The present invention generally relates to a catalyst for exhaust gas purification use.
As a catalyst for purifying the exhaust gases of an automobile or the like, there is known an item, wherein alumina powder is blended in aqueous solution including zirconium oxalate, cerium oxalate and noble metal catalytic components, the compound obtained after drying and burning the blended alumina powder is crushed by a ball mill together with water nitrate diluted to produce slurry liquid, whereby a coating layer is formed on honeycomb carrier, as disclosed in Japanese Laid-open Patent Publication (unexamined) Tokkaisho No. 59-209646.
In the catalyst, zirconium oxide, cerium oxide and noble metal catalytic component are contained in alumina coated layer of the honeycomb carrier surface. In this case, the zirconium oxide is a thermally stable component to prevent the noble metal catalytic component from being sintered (i.e., aggregation coarseness), with the cerium oxide assuming a function as oxygen storing component in such a manner that oxygen in the exhaust gas with oxygen concentration being high is absorbed, and, when the oxygen concentration becomes lower, the oxygen is released to stabilize the catalytic reaction.
As the other catalyst, an alumina coated layer is formed on the honeycomb carrier, which is dipped in a solution containing zirconium oxalate, cerium oxalate, noble metal catalytic component, and so on, and, then is picked up, dried, and baked, as disclosed in Japanese Laid-open Patent Publication (unexamined) Tokkaisho No. 56-87430. The coating layer of a mixture between alumina and zirconium oxide is formed on a catalyst carrier, which is dipped in a solution containing cerium nitrate or lanthanum nitrate and noble metal catalytic component, and, then is pulled up, dried and baked, as disclosed in Japanese Patent Publications (unexamined) Tokkaisho Nos. 61-197036 and 61-197037.
In each of the above-described conventional procedures, zirconium oxide and noble metal catalytic component are contained in the alumina coat layer, with each of them being independently dispersed, and not in positively close positional relation. Thus, the zirconium oxide cannot contribute sufficiently towards the sintering prevention of the noble metal catalytic component considering the amount thereof. Also the cerium oxide is aggregated at high temperatures, and is likely to be made coarse so as to lower the oxygen storing effect.
Also, the alumina of the alumina coating layer is in .gamma.--alumina condition of high activity at the beginning, and becomes finally stable .alpha.--alumina, with crystal phase transition being caused with the heat, so that the surface area is also reduced to lower the catalytic activity.